Electromagnet assembly

ABSTRACT

An electromagnet assembly has an inner magnet, an outer magnet, arranged around the inner magnet with an annular region extending between the inner magnet and the outer magnet, and a number of support elements extending through the annular region and dividing the annular region into a number of annular segments. The support elements are distributed in the annular region so as to form a small annular segment and a large annular segment.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to an electromagnet assembly.

In particular the disclosure is concerned with an electromagnet assemblyfor use in a medical imaging device.

Description of the Prior Art

In Magnetic Resonance Imaging (MRI), which is a known medical imagingtechnique, a main electromagnet is used to generate a strong magneticfield. In order to contain the field, it is known to employ ActiveShielding wherein an additional shield electromagnet is used to “shield”the generated magnetic field. The main electromagnet and the shieldelectromagnet are configured in a particular spatial arrangement, withthe shield magnet arranged to coaxially surround the main magnet.

For active shielding this particular spatial arrangement between theelectromagnets needs to be maintained by a support structure. In certainconventional MRI scanners a number of support elements are used, whilein other conventional MRI scanners a helium containment structure isused for this purpose. In either case, the support structure is arrangedto be axisymmetric, reflecting the fact that generated electromagneticloads are axisymmetric.

In addition to electromagnetic loads, the electromagnets may experienceexternally-caused loads along any direction and, particularly, along aradial direction. For example, loads may be caused during transportationof the magnet assembly. It is therefore ensured that the supportstructure is sufficiently strong to also bear such loads along anyradial direction. These loads, in contrast to electromagnetic loadscaused by interaction between the electromagnets, may cause local stressand strain on the magnets leading to a deformation and, ultimately,degradation in performance.

The above considerations result in an electromagnet assembly ofsubstantial weight and cost. Hence an electromagnet assembly which hasan optimal support structure, configured to provide adequate support intransit and in use while being lighter and using less materials thanprovided in examples of the related art is highly desirable.

SUMMARY OF THE INVENTION

The electromagnet assembly according to the invention has an innermagnet, an outer magnet arranged around the inner magnet with an annularregion extending between the inner magnet and the outer magnet, and anumber of support elements extending through the annular region anddividing the annular region into a number of annular segments. Thesupport elements are distributed in the annular region so as to form afirst annular segment and a second annular segment, with the firstannular segment being smaller than the second annular segment.

The second annular segment may be approximately an integer multiple ofthe first annular segment.

Each annular segment may be approximately an integer multiple or aninteger factor of another annular segment.

The outer magnet may have a number of outer support points, and theinner magnet may have a number of inner support points, with each of thesupport elements extending between an outer support point and an innersupport point.

Each pair of adjacent inner support points may span a distance, witheach distance spanned by a first pair of adjacent inner support pointsbeing an integer multiple or an integer factor of a distance spanned bya second pair of adjacent inner support points.

Each pair of adjacent outer support points may span a distance, witheach distance spanned by a first pair of adjacent outer support pointsbeing an integer multiple or an integer factor of a distance spanned bya second pair of adjacent outer support points.

The number of support elements may be at least three support elements.

A first support element may be located in a first half of the annularregion. A second and a third support element may be located in a secondhalf of the annular region.

The support elements may be substantially coplanar.

The outer magnet may have a first coil and a second outer magnet coil,which is spaced apart from the outer magnet first coil, wherein thefirst coil is configured to define the first annular region, and thesecond coil defines a second annular region that extends between theinner magnet and the second coil defined by the second coil. A number ofsupport elements may extend through the second annular region.

There may also be provided a medical imaging device comprising theelectromagnet assembly as set out above.

The inventive electromagnet assembly of the medical imaging deviceprovides greater support to the outer magnet along a vertical directionof the medical imaging device than along a horizontal direction of themedical imaging device.

The electromagnet assembly may have a larger number of support elementson a first side of the medical imaging device than on a second side ofthe medical imaging device.

The electromagnet assembly according to the invention has an optimizedsupport structure, designed to provide adequate support in transit andin use while being lighter and using less materials than conventionalassemblies of this type.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a known electromagnet assembly.

FIG. 2 shows schematic cross-sectional views of an electromagnetassembly according to the present invention.

FIG. 3 is a perspective view of an electromagnet assembly according tothe present invention.

FIG. 4 is an axial view of another example of an electromagnet assemblyaccording to the present invention.

FIG. 5 is a schematic cross-sectional view of another embodiment of thesupport structure according to the invention.

FIG. 6 is a schematic cross-sectional view of another embodiment of thesupport structure according to the invention.

FIG. 7 is a schematic cross-sectional view of another embodiment of thesupport structure according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a schematic illustration of a known electromagnet assembly100. According to the present example, the electromagnet assembly in useforms part of a Magnetic Resonance Imaging (MRI) scanner. For such apurpose the electromagnet assembly may be contained within a housingwhich, in operation, contains an inert gas as a coolant, for examplehelium. Hence the housing forms a cryogen vessel, which enables theelectromagnet assembly to be cooled to sufficiently low temperatures tooptimize performance.

The electromagnet assembly 100 is substantially cylindrical, defining anassembly axis A:A, a radial direction and a circumferential direction.The electromagnet assembly extends lengthwise along the assembly axis,and possesses rotational symmetry about the assembly axis.

On the left side of FIG. 1, an axial cross-section is shown. That is, aview showing a radial plane perpendicular to the assembly axis A:A. Onthe right-hand side of FIG. 1, a radial cross-section is shown. That is,a view showing a plane parallel to and including the assembly axis.

The electromagnet assembly 100 is designed to generate a magnetic fieldand to actively shield the magnetic field when generated. Theelectromagnet assembly has an inner (electro-) magnet 110 and an outer(electro-) magnet 120.

The inner magnet 110, or main coil assembly 110, has a hollowcylindrical shape. A plurality of coils disposed on the inner magnet,spaced apart from each other between a first axial end 112 and a secondaxial end 114, and generates a magnetic field.

The outer magnet 120, or shield coil assembly 120, bounds (or“surrounds”) the inner magnet 110. The outer magnet 120 is radiallylarger than the inner magnet and has an annular/ring shape. The innermagnet 110 and the outer magnet 120 are arranged coaxially and thusdefine a common assembly axis A:A. That is, axial cross-sections of theinner magnet 110 and the outer magnet 120 are concentrically arranged.

The outer magnet 120 is designed for active shielding, which involvesgenerating a magnetic field to partially cancel the magnetic fieldgenerated by the inner magnet. The outer magnet may comprise a pair ofcoils 122, 124. In line with the discussed example, the coils 122, 124are also referred to as “shield” coils 122, 124. A first shield coil 122and a second shield coil 124 are provided, the first shield coil 122being spaced apart from the second coil 124. More particularly, thefirst shield coil 122 is located toward the first end 112 of the innermagnet 110, and the second shield coil 124 is located toward the secondend 114 of the inner magnet.

An annular region 130 or annular gap 130 is formed between the innermagnet 110 and the outer magnet 120. More particularly, the annularregion is formed between an outer boundary (or “periphery”) 115 of theinner magnet and an inner boundary (or “periphery”) 125 of the outermagnet. Accordingly, the annular region 130 has a radial size determinedby an inner radius extending to the outer boundary of the inner magnet110 and an outer radius extending to the inner boundary of the outermagnet 120.

A support structure having a number of support elements 140 is locatedin the annular region 130. The support elements 140, or structuralrestraints, are provided between the inner magnet 110 and the outermagnet 120 to thereby hold the magnets 110, 120 in a substantially fixedposition relative to each other. The support elements 140 provide andmaintain precise positioning of the inner magnet 110 and the outermagnet 120. The support elements 140 thus provide rotational and axialsupport. The support elements may extend radially (with respect to theassembly axis A:A) between the inner magnet and the outer magnet. In theexample shown in FIG. 1, the support elements 140 are arranged to begenerally perpendicular to the inner magnet 110 and the outer magnet120, when viewed along an axial direction or, equivalently, an axialcross-section.

The number of support elements 140 extends through the annular region130 and thus divides the annular region 130 into a number of annularsegments 132. In the example of FIG. 1, there are eight annular segments132. The annular segments 32 are arranged uniformly, in that they are ofequal size and possess equal spacing.

More particularly, the annular segments 132 are segments of the annularregion 130 defined by the support elements 140 and, thus,circumferentially spaced. Accordingly, the annular segments 132 extendcircumferentially, their extent delimited by the support elements 140.That is, an annular segment 132 has a first circumferential end definedby a first support element 140, and a second circumferential end definedby a second support element 140. The annular segment 132 is thus boundedby a pair of adjacent circumferentially-spaced support elements 140.

The improved support structure according to the invention has a numberof support elements 240 distributed to define differently sized annularsegments 232, 234 so that least one small annular segment 232 is formed,and at least one large annular segment 234 is formed. That is, thesupport elements 240 are circumferentially-spaced with non-uniformspacing. This has the benefit of reducing weight and cost of the supportstructure, as some support elements 240 may be omitted relative to theexample of FIG. 1, while at the same time providing full support againstexpected loads and, particularly, loads along the radial direction.

FIG. 2 shows two schematic cross-sectional views of an electromagnetassembly 200 according to the present disclosure, while FIG. 3 shows aperspective view.

Some features of the assembly 200 are common to those of the knownassembly 100, and hence are not described in any further detail. Inparticular, the electromagnet assembly 200 comprises an inner magnet210, an outer magnet 220 arranged about the inner magnet, an annularregion 230 extending between the inner magnet and the outer magnet 220,and a number of support elements 240 extending through the annularregion 230 and dividing the annular region into a number of annularsegments 232, 234.

A first pair of adjacent support elements 240 delimits a first annularsegment 232. The adjacent support elements 240 are thuscircumferentially spaced apart from one another (and hence may be termed“adjacent” or “neighboring” one another). Similarly, a second pair ofadjacent support elements 240 delimits a second annular segment 234.

The first annular segment 232 is smaller than the second annular segment234. That is to say, the first pair of adjacent support elements 240 iscircumferentially spaced apart by a smaller distance than the secondpair of adjacent support elements 240. Thus, the first annular segment232 extends a shorter distance around the circumference of annular gap230 than the second annular segment 234.

The electromagnet assembly 200 has the inner magnet 210, the outermagnet 220 arranged around the inner magnet, and an annular region 230extending between the inner magnet 210 and the outer magnet 220. Thenumber of support elements 240 extends through the annular region 230and divides the annular region 230 into a number of annular segments232, 234. Support elements 240 are distributed along the annular regionto form the first annular segment 232 and the second annular segment234. The first annular segment 232 is smaller than the second annularsegment 234.

Generally, a single support element 240 has two adjacent (or“neighboring”) support elements 240. Thus each support element 240 formsa pair of support elements with each of the adjacent support elements240. That is, an individual support element 240 may belong to two pairssuch as, in the example shown in FIG. 2, there is a support element 240which belongs to the first pair of support elements 240 delimiting thesmall (“first”) annular segment 232 and also to the second pair ofsupport elements delimiting the large (“second”) annular segment 234.Some or all of the support elements 240 may be arranged to be coplanar.According to the present example, two support elements 240 are coplanarwith respect to a plane perpendicular to the assembly axis A:A.

The spacing of the support elements 240 is predetermined, at least inpart, to maintain a desired spacing of the inner magnet 210 and outermagnet 220 during operation. The spacing of the supports 240, and hencerelative extent of the first annular segment 232 and second annularsegment 234, is thus chosen with this aim.

In the example of FIG. 2 the large annular segment 234 is a wholeinteger multiple of the small annular segment 232.

An alternative example is shown in FIG. 4. The example of FIG. 4 isidentical to that of FIGS. 2, 3 except it comprises an extra supportelement 240′, which defines a third annular segment 332 and a fourthannular segment 334, where the third annular segment 332 is smaller thanthe fourth annular segment 334. However, as shown in the further exampleof FIG. 4, the extent of the large annular segment 334 is less than awhole integer multiple of the small annular segment 332. That is to say,the extent of the large annular segment 334 is greater than the extentof the small annular segment 332, but less than twice the extent of thesmall annular segment 332.

Alternatively (not shown) the extent of the large annular segment 334may be greater than a whole integer multiple of the small annularsegment 332. That is to say the extent of the large annular segment 334may be greater than twice the extent of the small annular segment 332.

Put another way, in the examples of FIGS. 2 to 4, the large annularsegments 234, 334 are greater than the extent of the small annularsegment 332, and may be substantially greater than or smaller than awhole integer multiple of the small annular segments 232, 332.

In the example of FIGS. 2, 3 the support elements 240 are symmetricallyarranged to either side of an axis B:B perpendicular to axis A:A suchthat the spacing of the support elements 240 is the same on either sideof the axis B:B. In other examples, for example as shown in FIG. 4, someof the supporting elements 240 may be circumferentially spaced apart todifferent extents, for example such that the spacing of the supportelements 240 is different on either side of the axis B:B, and/or suchthat the spacing of the support elements 240 varies around the annularregion 230.

With reference to the example of FIGS. 2 to 4 a number of supportelements 240 of the assembly 200 are omitted relative to the example ofFIG. 1. Hence some annular segments are integer multiples of other,smaller annular segments.

In the example of FIGS. 2, 3 a first half of the annular region 230includes a single support element 240, while a second half of theannular region 230 includes three support elements 240. According to thepresent example, the first half is an upper half while the second halfis a lower half of the annular region 230. That is to say, the “upperhalf” is shown in the figures as above the axis A:A and the “lower half”is below the axis A:A. This may correspond to the orientation of thedevice in use, where the “upper half” is spaced apart from a supportingstructure (e.g. a floor) by the “lower half”. A first support element240 is located in first half of the annular region 230, and at least twosupport elements 240 are in a second half of the annular region 230.

In the example of FIGS. 2, 3, the large annular segment 234 isapproximately three times larger than the small annular segment 232,wherein the individual widths of the support elements 240 (as measuredalong the circumferential direction) have been disregarded. That is, thelarge annular segment 234 has a circumferential length, i.e. a length asmeasured along the circumferential direction, which is approximatelythree times larger than a circumferential length of the small annularsegment 232. More precisely, the large annular segment 234 issubstantially three times the small annular segment 232 plus two timesthe width of a single support element 240.

Every pair of adjacent support elements 240 spans an angle. Disregardingthe width of the individual support elements 240, in the example ofFIGS. 2, 3 the small annular segment 232 corresponds to an angle of 45°(or “deg”, denoting degree of an arc), and the large annular segment 234corresponds to an angle of 135°.

The electromagnet assembly of the present disclosure comprises three ormore support elements 240, wherein a first angle is spanned by a firstpair of adjacent support elements 240, a second angle is spanned by asecond pair of adjacent support elements 240, and the first anglediffers from the second angle.

The support elements 240 are fixed to the inner magnet 210 and the outermagnet 220 by suitable means, for example mechanically. The inner magnet210 may have a number of inner support points 215, and the outer magnet220 comprises a plurality of outer support points 225. According to thepresent example, an individual support element 240 extends from an innersupport point 215 to an outer support point 225, and according to someexamples each support element 240 may extend between an outer supportpoint 225 and an inner support point 215.

Each pair of adjacent inner support points 215 spans a distance. Thisdistance is measured along the outer boundary of the inner magnet 210and, in particular, along the circumferential direction. Each distancespanned by a first pair of adjacent inner support points 215 is aninteger multiple or an integer factor of a distance spanned by a secondpair of adjacent inner support points 215. Conveniently, either theinteger multiple or the integer factor may exclude the integer beingequal to unity.

Similarly, each pair of adjacent outer support points 225 spans adistance. According to some examples, each distance spanned by a firstpair of adjacent outer support points 215 is an integer multiple or aninteger factor of a distance spanned by a second pair of adjacent outersupport points 225.

According to the present example of FIGS. 2, 3, each annular segment232, 234 approximately corresponds to an integer multiple or an integerfactor of another annular segment 232, 234. Each large annular segment234 may be an integer multiple of any one of the small annular segments232. Similarly, each small annular segment 232 may be an integer factorof any large annular segment 234.

The support elements 240 are arranged to be substantially perpendicularto the inner magnet 210 as well as the outer magnet 220, when viewedalong an axial direction. The support elements need not, however, beperpendicular to the axis A:A, as shown in FIG. 2.

Similar to the known assembly 100, the inner magnet 210 comprises afirst (axial) end 212 and a second (axial) end 214, which delimit theinner magnet along the assembly axis A:A. Analogously, the outer magnet220 comprises a first (axial) end 226 and a second (axial) end 228,which delimit the outer magnet along the assembly axis.

The electromagnet assembly 200 is a structure for generating a highmagnetic field under superconducting conditions, and is therefore of aprecise design and manufactured within small tolerances. When theassembly is subjected to shock or vibration loads, it is important tomitigate these loads in order to prevent, for example, distortion of therelative positions of the inner magnet 210 and the outer magnet 220 tomaintain performance of the assembly.

During transportation of the electromagnet assembly 200, or a devicecomprising the assembly, the most significant shock loading may in thevertical orientation. To manage the distortion and stress exhibited bythe magnets due to vertically oriented loading, the support elements 240are concentrated in a portion of the annular region 230 corresponding toa vertical region, either the lower vertical region or the uppervertical region. This causes the loads to be dominated by tension orcompression of the support elements, rather than bending. At the sametime, support elements carrying non-significant support loads areremoved from the electromagnet assembly to reduce the mass and cost ofthe structure.

Moreover, ferromagnetic materials may be present around theelectromagnet assembly 200 when installed. These ferromagnetic materialsmay serve various purposes, for example as structural features of abuilding or magnetic shielding. However, electromagnetic attractionbetween the electromagnet assembly and the ferromagnetic materials maybe caused, which in turn can cause local stress and strain of theelectromagnet assembly. If the assembly is unsupported against suchattraction, a degradation in performance can result. Hence supportelements may be distributed in the annular region 230 dependent on theproperties of the building or other structure in which the electromagnetassembly is to be installed. For example, a number of support elements240 may be closely spaced along a horizontal direction, with respect thedevice in which the assembly is contained, in order to support againsthorizontal attraction to a ferromagnetic building wall/floor.

Furthermore, considerations relating to ferrous materials in surroundingstructures become increasingly important with increasing magnetstrength. High and ultra-high magnetic fields generated by currentelectromagnet technology may exceed 7 T (tesla) and may therefore bevery susceptible to distortions resulting from attraction between themagnets and surrounding ferrous material. An increasingly complex andmassive support structure, however, is advantageously avoided becauseotherwise the electromagnet assembly and/or a device, such as an MRIscanner, have to be shipped in parts, assembled and cooled. Thispotentially adds weeks of delay between shipping and operation of thedevice and may add substantial costs. By contrast, the electromagnetassembly 200 according to the present disclosure provides for asufficiently strong support between the magnets and, at the same time,reduced weight. An MRI scanner, or other device, comprising the assemblymay therefore be shipped as a whole unit.

The electromagnet assembly 200 as shown in FIG. 2 has support elements240 which are arranged non-uniformly, whereas according to the knownexample of FIG. 1 support elements 140 are arranged uniformly.Non-uniformity of the distribution of support elements is not intendedto preclude rotational symmetry. Although the electromagnet assembly 200does not possess rotational symmetry, it is envisaged that certainexamples may. Such example assemblies may be symmetric with respect to arotation of, for example, 120° or a rotation of 180°.

As shown in FIG. 3, the electromagnet assembly 200 comprises the outermagnet 220 comprises a pair of coils 222, 224. A first coil 222 islocated towards the first end 212 of the inner magnet 210, and a secondcoil 224 is located towards the second end 214 of the inner magnet. Aplurality of axial reinforcing elements 250, or reinforcement members250, extends between the pair of coils. The reinforcing members 250provide additional support of the outer magnets 220, particularlyagainst forces along the assembly axis A:A which may includeelectromagnetic forces exerted as part of active shielding. In suchexamples, where the coils are configured for active shielding, the coilsmay alternatively be referred to as “shield” coils 222, 224.

Conveniently, the number of reinforcing members 250 is greater than thenumber of radial support elements 240. According to the present example,there are eight reinforcing members 250. The spacing between a pair ofadjacent reinforcing members 250 may correspond to the small annularsegment 232.

An annular region 230, 231 is defined by each shield coil 222, 224 andthe inner magnet 210. Two sets of support elements 240 are provided,each set supporting a different shield coil 222, 224 relative to theinner magnet 210. That is to say, the first shield coil 222 defines afirst annular region 230 and the second shield coil 224, which isaxially spaced from the first shield coil 222, defines a second annularregion 231 which extends between the inner magnet 210 and the secondcoil 224. A set of support elements 240 is provided in each annularregion 230, 231 to provide support to the corresponding shield coil 222,224.

FIGS. 6 and 7 show alternatives for providing radial support to theelectromagnet assembly 200. According to the earlier example,illustrated in FIG. 5, radial supports are provided such that they arespaced apart axially (i.e. along the Axis A:A). Alternatively, as shownin FIGS. 6 and 7, radial support may be provided as a single structure.In such an example, a single annular region 240 is considered to bedefined, including both outer magnets 220.

According to another alternative, shield coil support may be providedthrough local fixings, shown in FIG. 5, or a journal, shown in FIG. 6.

Although the examples discussed above have four support elements 240, itis envisaged that any plurality of support elements of at least threemay be used in an electromagnet assembly.

A medical imaging apparatus according to the invention has a medicalimage data acquisition scanner with an electromagnet assembly 200, asdescribed above, therein. The electromagnet assembly 200 is arranged toprovide greater support to the outer magnet 220 along a verticaldirection (e.g. Axis B:B) of the medical imaging scanner than along ahorizontal direction (e.g. Axis A:A) of the medical imaging device.

Hence the medical imaging scanner may be considered as having a “firstside” (which may be the “lower half” described above) and a “secondside” (which may be the “upper half” described below). Thus the firstside may be opposite to the second side across the axis A:A to definevertical sides. Put another way, the first side may be separated fromthe second side by a horizontal axis.

Alternatively the first side may be opposite to the second side acrossthe axis B:B to define “horizontal sides”. That is to say, the firstside may be separated from the second side by a vertical axis.

Hence the first side and the second side are “sides” delimiting thedevice along the radial direction of the electromagnet assembly.

Hence according to the present invention, the medical imaging apparatusis designed so that the electromagnet assembly has a larger number ofsupport elements 240 on the first side of the scanner than on the secondside, or has a fewer number of support elements 240 on the first sidethan on the second side.

According to an embodiment, the electromagnet assembly has a greaternumber of support elements 240 on a first horizontal side of the scannerthan on a second horizontal side.

The support elements 240 on the first horizontal side may be orientatedto support along the vertical direction rather than along the horizontaldirection.

Generally, the structure of an electromagnet assembly is dominated bythe large electromagnetic loads generated by the electromagnets, and isdesigned and constructed to be axisymmetric. Any directional loadconditions are assumed to act in any direction, and hence designs aremade axisymmetric. As structures are made more efficient as to materialand cost, non-axisymmetric loading becomes increasingly significant.Conventional active shield coil support is achieved through radialsupport elements, or use of the helium containment structure, both ofwhich are axisymmetric. Both of these structures are inefficient onmaterial and cost, since they provide full support at a level suitablefor the high vertical load conditions. By contrast, the electromagnetassembly 200 according to the present disclosure improves on theseconsiderations.

The electromagnet assembly is designed to provide adequate support intransit and in use. Expected loads when in use, for example an externalload generated through interaction with the site of operation of theelectromagnet, can be accommodated by including support elements 240arranged to support against these expected loads. Such an arrangementmay be particularly desirable where large loads are caused along aparticular radial direction of the electromagnet assembly, for exampledue to the presence of ferromagnetic material in a supporting floor, oradjacent wall, ceiling or other nearby structure

Support elements 240 are omitted from regions where it is expected nosupport against loads on the electromagnets is required. The resultingelectromagnet assembly is therefore less complex, simplifying itsmanufacturing and reducing the amount of materials required for itsmanufacture. Hence the resulting electromagnet assembly is generallylighter, which may be advantageous for transportation as well asoperation of the electromagnet assembly.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the Applicant to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of the Applicant's contribution to theart.

The invention claimed is:
 1. An electromagnet assembly for generating astrong magnetic field under superconducting conditions, comprising: aninner magnet; an outer magnet around the inner magnet, with an annularregion extending between the inner magnet and the outer magnet; aplurality of support elements extending through the annular region anddividing the annular region into a plurality of annular segments; andsaid support elements being distributed in the annular region so as toform a first annular segment and a second annular segment, with thefirst annular segment being smaller than the second annular segment,wherein the outer magnet comprises: a first coil; a second coil spacedapart from the first coil; the first coil defining a first annularregion; the second coil defining a second annular region that extendsbetween the inner magnet and the second coil; and a plurality of supportelements through the second annular region.
 2. The electromagnetassembly according to claim 1, wherein the second annular segment is aninteger multiple of the first annular segment.
 3. The electromagnetassembly according to claim 1, wherein each annular segment is aninteger multiple or an integer factor of another annular segment.
 4. Theelectromagnet assembly according to claim 1, wherein: the outer magnetcomprises a plurality of outer support points; the inner magnetcomprises a plurality of inner support points; and each of the supportelements extends between an outer support point and an inner supportpoint.
 5. The electromagnet assembly according to claim 4, wherein: eachpair of adjacent inner support points spans a distance; and eachdistance spanned by a first pair of adjacent inner support points is aninteger multiple or an integer factor of a distance spanned by a secondpair of adjacent inner support points.
 6. The electromagnet assemblyaccording to claim 4, wherein: each pair of adjacent outer supportpoints spans a distance; and each distance spanned by a first pair ofadjacent outer support points is an integer multiple or an integerfactor of a distance spanned by a second pair of adjacent outer supportpoints.
 7. The electromagnet assembly according to claim 1, wherein theplurality of support elements comprises at least three support elements.8. The electromagnet assembly according to claim 1, wherein: a firstsupport element is located in a first half of the annular region; and asecond support element and a third support element are located in asecond half of the annular region.
 9. The electromagnet assemblyaccording to claim 1, wherein the support elements are coplanar.
 10. Amedical imaging apparatus comprising: a medical image data acquisitionscanner; an electromagnetic assembly in said scanner that generates astrong magnetic field under superconducting conditions; and saidelectromagnetic assembly comprising an inner magnet, an outer magnetaround the inner magnet, with an annular region extending between theinner magnet and the outer magnet, a plurality of support elementsextending through the annular region and dividing the annular regioninto a plurality of annular segments, and said support elements beingdistributed in the annular region so as to form a first annular segmentand a second annular segment, with the first annular segment beingsmaller than the second annular segment, wherein the outer magnetcomprises: a first coil; a second coil spaced apart from the first coil;the first coil defining a first annular region; the second coil defininga second annular region that extends between the inner magnet and thesecond coil; and a plurality of support elements extending through thesecond annular region.
 11. The medical imaging apparatus according toclaim 10, wherein the electromagnet assembly provides greater support tothe outer magnet along a vertical direction of the medical imagingscanner than along a horizontal direction of the medical imagingscanner.
 12. The medical imaging apparatus according to claim 10,wherein the electromagnet assembly has a larger number of supportelements on a first side of the medical imaging scanner than on a secondside of the medical imaging scanner.